Hardenable stainless steel alloy



Patented Apr. 14, 1953 HARDENABLE STAINLESS STEEL ALLOY Norman S. Mott, Westfield, N. J assigiior to The Cooper Alloy Foundry 00., Hillside, N. J a corporation of New Jersey No Drawing. Application July 11, 1950, Serial No. 173,239

9 Claims. 1

This invention relates to stainless steel alloys,

and more particularly to such an alloy which is hardenable.

A most popular stainless steel alloy is a high chromium nickel steel, especially the so-called 18-8 stainless steel. This stainless steel is not hardenable, and many people have worked for years in an effort to make such a stainless steel which will be hardenable. One prior result has required the addition of a very substantial quantity f beryllium, but beryllium is costly, and the alloy has therefore not been available for commercial applications where .cost is a factor. Moreover, the beryllium greatly lowered the corrosion resistance of the alloy. Another proposed alloy uses copper hardening. It requires no beryllium, but is not an 18-8 steel for it is characterized by a greatly reduced nickel content, made up largely by the addition of cop-per as a hardening agent. This alloy may also be made with a small amount of beryllium. However, in either case, it has substantially less corrosion resistance than the ordinary 18-8 stainless steel with its higher nickel content.

The primary object of the present invention is to overcome the foregoing difliculties, and to provide a low cost, hardenable, chromium nickel stainless steel alloy of highcorrosion resistance.

My alloy employs beryllium, but I have found I that by. control of the chemical composition of the alloy, or more specifically by the addition of a substantial quantity of silicon and molybdenum, the amount, of beryllium required is reduced to so small an amount as not to add excessively to the cost of the alloy. The resulting alloy is soft enough to be readily machinable in the quench-annealed condition. It may be precipitation hardened by heat treatment at a comparatively low temperature. This heat treatment produces an increase in hardness such as to make the alloy quite resistant to galling and erosioncorros'ion, thus making it useful for such purposes as valve discs, pump impellers, and the like, where these characteristics of hardness coupled with high corrosion resistance are needed.

My new alloy may be made with or without copper. It has good corrosion resistance without copper, but I have found that the addition of copper improves the corrosion resistance of the alloy, particularly itsresistance to sulphuric acid,

. 2 without at the same time materially affecting the hardenability of the alloy one way or the other.

The base material may be a conventional 18-8 stainless steel, this having the usual range of 5 from 17% to 21% chromium and 7% to 14% nickel. Hitherto beryllium hardening required very large additions of beryllium, which made the alloy exceedingly costly, and which resulted in a very low resistance to corrosion. By control of the composition I have found it possible to produce a corrosion resistant and hardenable 18 and 8 type of alloy by the addition of relatively small amounts of beryllium. In accordance with my invention, silicon, molybdenum and beryllium are added to produce a silicon content in a range of from 2% to 4%, a molybdenum content in a range of from 1.5% to 4%, and a beryllium content in a range from 0.05% to 0.20%. If the alloy is to be made with copper in order tofurther increase the corrosion resistance, copper may be added to provide a range of from 1% to 4% of copper.

Three specific examples of my alloy with their resulting hardness are given hereinafter. These alloys included copper. The chemical analyses (by weight) of three alloys were:

Table 1 Example- 3. 30 s. so

In these tables WQ means water quenched, and PH means precipitation hardened after water quenching.

In the foregoing analyses the iron content is not included, but it is, of course, understood that subject to the presence of small quantities of other elements, such as the iiiipuritie's' found in steels, the balance is iroii. To cover this situa- 3 tion I may state that in addition to the elements named in the analyses, the remainder is substantially all iron.

When these alloys were heat treated at 2,000 E. for one hour, followed by water quenching, they had a Brinell hardness as indicated above. The water quenched material was further heat treated at 925 F. for eight hours, followed by furnace cooling, a treatment known as precipitation hardening. This resulted in a greatly increased Brinell hardness, as indicated above. For purposes of comparison it may be pointed out that the hardness obtainable in a standard 18-8 SMO, Type 316, A. I. S. alloy as cast is a Brinell hardness of 180.

The alloy may be given a still greater hardness by appropriate heat treatment. Thus the alloy of Example 1 was heated at 2,250 F. for one hour and water quenched (instead of 2,000 F. as first specified), and the material was then precipitation hardened as specified above, that is, it was heat treated at 925 F. for eight hours, followed by furnace cooling. This treatment produced a Brinell hardness of 444.

Under corrosion test the number 1 alloy, as water quenched, when immersed in 50% sulphuric acid at room temperature, had a corrosion rate of 0.00003 inch penetration per month.

After precipitation hardening (following water quenching as described above) it was 0.00010 1. P. M. For purposes of comparison it may be pointed out that the corrosion rate for a standard 18-8 SMO, Type 316, A. I. S. I. alloy was 0.11300 I. P. M.

The corrosion resistance was also measured by immersing specimens in hydrochloric acid at room temperature. The corrosion rate after water quenching and without precipita-- tion hardness was 0.00081 I. P. M. The corrosion rate for both water quenching and precipitation hardening as described above was 0.00100 1. P. M. For purposes of comparison it may be pointed out that the corrosion rate for a standard 18-8 SMO, Type 316 A. I. S. I. alloy was 0.04135 1. P. M.

Various other corrosion tests were carried out on the precipitation hardened number 1 alloy. The results of these tests are set forth in the following table Table II I. P. M. 20% HNO3 boiling .00011 10% H2504 boiling .03500 71% HNOs 70 F .00024 80% I-INO3+20% H2SO4@130 F .00006 I-INOs+40% I-IzSO4@130 F .00003 20% HNO3+80% H2SO4@ 130 F .00004 20% I-INO3+60% H2SO4+20% V V H2O@130 F .00004 The alloy is also fully resistant to salt spray.

copper, the following alloys were employed, these.

Table III Example- These examples show that the 18-8 SMO type of alloy can be hardened by the addition of a small quantity of beryllium without the presence of copper, provided that the silicon content is kept high, say, over 2%. One known hardenable alloy hardens by the addition of copper. My alloy hardens by the addition of a small amount of beryllium, and makes this possible by increasing the silicon and the molybdenum, particularly the former. By comparing Table I with Table II, it will be seen that with my alloy the addition of copper has no significant effect on hardening. I find that copper has no hardening effect when the nickel content is kept high, as is done in my alloy. In the prior alloy which hardened by the addition of copper the nickel content was cut down drastically. My alloy is beryllium hardened with or without copper. The prior alloy was copper hardened with or without beryllium. It will be noted that the first of these alloys (Example 4) had a low beryllium content, and the second (Example 5) a higher beryllium content, thus showing that the alloy may be employed without copper in either case.

In order to test the effect of increasing or decreasing the beryllium content the following alloys were employed:

Table IV Example- Example 6 having no beryllium shows that high hardness is not obtained without the addition of at least a small percentage of beryllium. There was a high percentage of copper, yet the alloy was not hardened, showing that the alloy of my invention is beryllium hardened, and not copper hardened.

Example 7 shows that beryllium in an amount less than 0.05% does not produce appreciable hardening. Example 8 shows that more than 0.05% beryllium does harden. The beryllium in Example 7 was 0.030, which was inadequate, and the beryllium in Example 8 was 0.072, which was adequate. Example 9. shows the use of a substantially higher amount of beryllium, specifically 0.380%. This greatly raised the hardness of the alloy after hardening, but also raised the Example-'- In Example 10, molybdenum was omitted. The alloy did not harden appreciably, and was no better than Example 6 discussed above in which beryllium was omitted. This shows that the presence of molybdenum is necessary.

In Example 11 the silicon content was kept below 2%, it being 1.16%, and no hardening was obtained despite the presence of an adequate beryllium content. In Example 12, the silicon again was less than 2%, it being 1.56%, and the alloy did not appreciably harden despite the presence of a high beryllium content of 0.350. This shows that a low silicon alloy will not harden even after considerably increasing the beryllium content. Example 13 had a silicon content over 2%, specifically, 2.32%, and hardened well with only a moderate beryllium content of 0.137%.

In all of the foregoing tables the abbreviation W Q means water quenched after one hour at 2,000 F. The abbreviation P H means water quenched after one hour at 2,000 F. followed by furnace cooling after eight hours at 925 F. (i. e., precipitation hardened).

In all cases it is to be assumed that the remainder of the alloy is substantially all iron (subject to impurities found in steels).

The maximum carbon content should be no higher than, say, 0.12%. The carbon content is preferably limited to a somewhat lower value of 0.08% for best ductility and corrosion resistance.

The alloy is weldable by using welding rods of the same general composition, but with an increased beryllium content. The beryllium content is increased about 50% in order to compensate for its loss during the welding operation. If the weld area itself is to be put into the hardened condition the complete heat treatment must follow the Welding operation.

It is believed that the composition and behavior of my improved hardenable stainless steel alloy, as well as the advantages thereof, will be apparent from the foregoing detailed description. The new alloy is low in cost, and high in corrosion resistance. It is soft enough in the quenchannealed condition to be readily machinable, and may be precipitation hardened by a comparatively low-temperature heat treatment. The increase in hardness makes the alloy resistant to galling and erosion. The alloy is resistant to salt spray and to various combinations of acids in diflerent concentration and at elevated tomperature as well as at room temperature. The alloy may be welded. The-alloy has the high nickel content of a regular 188 type stainless steel alloy, and retains substantially all of the advantages of that enormously and justly popular type of stainless steel.

It will be apparent that while I have set forth specific examples of my improved alloy, many changes ma be made, without departing from the scope of the invention as sought to be defined in the following claims.

I claim:

1. A precipitation hardenable alloy of the type known as 18 and 8 stainless steel with the usual range of from 7% to 21% chromium and 7% to 14% nickel, said alloy havingsilicon in a range of from 2% to 4%, molybdenum in a range of from 1.5% to 4%. beryllium in a range of from 0.05%- to 0.20%., the remainder being essentially iron with a carbon content not exceeding about 2. A precipitation hardenable alloy of the type known as '18 and a stainless steel with the usual range of from 17% to 21% chromium and 7% to 14% nickel, said alloy having silicon in a range of from 2% to 4%, molybdenum in a range of from 1.5% to 4%, beryllium in a range of from 0.05% to 0.20%, copper in a range of from 1% to 4% in order to increase the corrosion resistance of the alloy, and the balance being essentially all iron with a carbon content not exceeding about 0.12%.

3. A precipitation hardenable alloy of the type known as 18 and 8 stainless steel, said alloy having approximately the following chemical analysis: carbon .063% chromium 18.10%; nickel 9.65%; copper 2.64%; molybdenum 2.64%; beryllium 065%; silicon 2.56%; manganese .47%, and the balance of the alloy being substantially all iron.

4. A precipitation hardenable alloy of the typeknown as 18 and 8 stainless steel, said alloy having approximately the following chemical analysis: carbon .050%; chromium 19.18%; nickel 10.25%; copper 3.20%; molybdenum 2.84%; beryllium .099%; silicon 8.30%; manganese 57%, and the balance of the alloy being substantially all iron.

5. A precipitation hardenable alloy of the type known as 18 and 8 stainless steel, said alloy having approximately the following chemical analysis: carbon 063% chromium 18.10%; nickel 9.54%; copper 3.60%; molybdenum 2.64%; beryllium 106%; silicon 3.80%; manganese 1.10%, and the balance of the alloy being substantially all iron.

6. A low cost hardenable stainless steel of high corrosion resistance consisting essentially of a chromium nickel stainless steel of the type known generally as 18 and 8 having added thereto beryllium, silicon, and molybdenum, the beryllium ranging from 0.05% to 0.20%, the silicon ranging from 2% to 4%, and the molybdenum ranging from 1.5% to 4%, the said alloy being soft enough to be readily machinable in the quench-annealed condition, and being adapted to be precipitation hardened by a comparatively low temperature heat treatment.

7. A low cost hardenable stainless steel of high corrosion resistance consisting essentially of a.

chromium nickel stainless steel of the type known generally as 18 and 8 having added thereto beryllium, silicon, molybdenum, and copper, the beryllium ranging from 0.05% to 0.20%, the silicon ranging from 2% to 4%, the molybdenum ranging from 1.5% to 4%, and the copper ranging from 1% to 4%, the said alloy being soft enough to be readily machinable in the quench-annealed condition, and, being adapted to be precipitation hardenedby a comparatively low temperature heat treatment.

8. A low cost hardenable stainless steel of high corrosion resistance consisting essentially of a chromium nickel stainless steel of the type known generally as 18 and 8 having added thereto beryllium, silicon, and, molybdenum, the beryllium ranging from 0.05% to 0.20%, the silicon ranging from 2% to 4%, and the molybdenum ranging from 1.5% to 4%, the remainder of the alloybeing substantially all chromium, nickel, and iron with a carbon content not exceeding about 0.12%, with said remainder being in usual proportions, the said alloy being soft enough to be readily machinable in the quench-annealed condition, and being adapted to be precipitation hardened by a comparatively low temperature heat treatment.

9. A low cost hardenable stainless steel of high corrosion resistance consisting essentially of a chromium nickel stainless steel of the type known generally as 18 and 8 having added thereto beryllium, silicon, molybdenum, and copper, the beryllium ranging from 0.05% to 0.20%, the silicon ranging from 2% to 4%, the molybdenum ranging from 1.5% to 4%, and the copper ranging from 1% to 4%, the remainder of the alloy being substantially all chromium, nickel, and iron with a carbon content not exceeding about 0.12%, with said remainder being in usual proportions, the said alloy being soft enough to be readily machinable in the quench-annealed condition, and being adapted to be precipitation hardened by a comparatively low temperature heat treatment.

' NORMAN S. MOTT.

References Cited in the file or this patent UNITED STATES PATENTS Number 

1. A PRECIPITATION HARDENABLE ALLOY OF THE TYPE KNOWN AS 18 AND 8 STAINLESS STEEL WITH THE USUAL RANGE OF FROM 17% TO 21% CHROMIUM AND 7% TO 14% NICKEL, SAID ALLOY HAVING SILICON IN A RANGE OF FROM 2% TO 4% MOLYBDENUM IN A RANGE OF FROM 1.5% TO 4%, BERYLLIUM IN A RANGE OF FROM 0.05% TO 0.02%, THE REMAINDER BEING ESSENTIALLY IRON WITH A CARBON CONTENT NOT EXCEEDING ABOUT 0.12%. 